﻿578 
  Dr. 
  C. 
  G. 
  Barkla 
  and 
  Mr. 
  C. 
  A. 
  Sadler 
  on 
  

  

  is 
  for 
  a 
  considerable 
  range 
  in 
  the 
  penetrating 
  power 
  of 
  the 
  

   primary 
  merely 
  proportional 
  to 
  the 
  ionization 
  produced 
  by 
  

   that 
  primary 
  in 
  a 
  thin 
  film 
  of 
  air 
  and 
  is 
  otherwise 
  independent 
  

   of 
  the 
  penetrating 
  power 
  of 
  the 
  primary. 
  This 
  is 
  not 
  

   general, 
  as 
  in 
  many 
  cases 
  — 
  probably 
  all 
  — 
  the 
  homogeneous 
  

   radiation 
  disappears 
  when 
  the 
  primary 
  radiation 
  becomes 
  

   more 
  absorbable. 
  

  

  The 
  intensity 
  of 
  the 
  homogeneous 
  rays 
  in. 
  a 
  given 
  direction 
  

   does 
  not 
  depend 
  appreciably 
  on 
  the 
  position 
  of 
  the 
  plane 
  of 
  

   polarization 
  of 
  primary 
  beam 
  producing 
  them. 
  

  

  The 
  intensity 
  of 
  this 
  radiation 
  in 
  a 
  direction 
  approximately 
  

   opposite 
  to 
  that 
  of 
  propagation 
  of 
  the 
  primary 
  beam 
  producing 
  

   it, 
  is 
  within 
  the 
  small 
  errors 
  of 
  experiment 
  equal 
  to 
  that 
  in 
  a 
  

   direction 
  at 
  right 
  angles 
  (g). 
  

  

  This 
  radiation 
  is 
  specially 
  penetrating 
  to 
  the 
  element 
  which 
  

   emits 
  it 
  and 
  to 
  a 
  less 
  extent 
  to 
  elements 
  of 
  neighbouring 
  

   atomic 
  weight 
  (/). 
  

  

  The 
  fraction 
  of 
  the 
  homogeneous 
  rays 
  from 
  one 
  element 
  — 
  

   copper 
  — 
  scattered 
  by 
  air, 
  is 
  the 
  same 
  as 
  that 
  for 
  X-rays 
  

   proceeding 
  direct 
  from 
  an 
  X-ray 
  tube 
  (within 
  experimental 
  

   errors) 
  (b). 
  

  

  The 
  absorbability 
  of 
  the 
  secondary 
  rays 
  from 
  copper 
  which 
  

   are 
  scattered 
  by 
  air 
  — 
  tertiary 
  rays 
  — 
  is 
  the 
  same 
  as 
  that 
  for 
  

   the 
  direct 
  secondary 
  (h). 
  

  

  Theory, 
  

  

  The 
  theory 
  of 
  the 
  scattered 
  X-rays 
  has 
  been 
  dealt 
  with 
  in 
  

   various 
  papers, 
  and 
  the 
  experimental 
  evidence 
  in 
  support 
  of 
  

   that 
  theory 
  — 
  briefly 
  referred 
  to 
  in 
  this 
  paper 
  — 
  is 
  so 
  over- 
  

   whelming 
  that 
  it 
  need 
  not 
  be 
  further 
  discussed 
  here. 
  

  

  It 
  is 
  important, 
  however, 
  to 
  consider 
  the 
  evidence 
  we 
  have 
  

   regarding 
  the 
  nature 
  and 
  origin 
  of 
  the 
  homogeneous 
  rays, 
  

   which 
  are 
  characteristic 
  of 
  the 
  elements 
  emitting 
  them. 
  

  

  The 
  fact 
  that 
  the 
  homogeneous 
  secondary 
  rays 
  from 
  copper 
  

   are 
  scattered 
  by 
  air 
  in 
  approximately 
  the 
  same 
  proportion 
  as 
  

   the 
  primary 
  rays 
  proceeding 
  direct 
  from 
  an 
  X-ray 
  tube, 
  and 
  

   that 
  the 
  absorbability 
  of 
  these 
  scattered 
  rays 
  is 
  the 
  same 
  as 
  

   that 
  of 
  the 
  direct 
  secondary 
  rays, 
  is 
  strong 
  evidence 
  that 
  they 
  

   are 
  of 
  the 
  nature 
  of 
  X-rays, 
  for 
  neither 
  the 
  observed 
  intensity, 
  

   nor 
  the 
  scattering 
  without 
  degradation, 
  would 
  have 
  been 
  ex- 
  

   pected 
  on 
  any 
  corpuscular 
  theory, 
  whereas 
  they 
  are 
  in 
  perfect 
  

   harmony 
  with 
  the 
  aether 
  pulse 
  theory. 
  

  

  The 
  relation 
  between 
  the 
  absorption 
  of 
  a 
  mixture 
  of 
  the 
  

   homogeneous 
  rays 
  from 
  a 
  number 
  of 
  elements 
  by 
  various 
  

   elements 
  and 
  the 
  atomic 
  weight 
  of 
  those 
  absorbing 
  elements 
  

   is 
  also 
  very 
  different 
  from 
  that 
  found 
  for 
  any 
  material 
  

  

  